This application claims priority under 35 U.S.C. §119(a) on Patent Application No. 2003-372778 filed in Japan on Oct. 31, 2003, the entire contents of which are hereby incorporated by reference.
The present invention relates to a sheet detecting apparatus for detecting a transport state of a sheet, which is provided in an image forming apparatus, and an image forming apparatus using the same.
Generally, in such an image forming apparatus, an electrostatic latent image is formed on an image carrier based on input image data, and the electrostatic latent image is developed by an development apparatus to form a development-agent image on the image carrier. The development-agent image formed on the image carrier is transferred to a transported sheet. Thereafter, the development-agent image is melted and fixed by heat and pressure onto the sheet using a fixation device.
When a positional displacement occurs between the development-agent image formed on the image carrier and the transported sheet due to the transport skew of the sheet, the position of the formed image differs from the position of a read original image. Such a positional displacement due to the transport skew of a sheet is usually inconsistent and also varies depending on a difference in the size or type of sheets or an accommodation means for accommodating sheets.
Therefore, when trying to form an image g on an entire sheet p with no margin (entire sheet image formation) as shown in FIG. 16, a positional displacement between a development-agent image and the sheet p would cause loss of a portion of the image g transferred to the sheet p, resulting in an unsightly image product.
To avoid this, taking into consideration the positional displacement between a development-agent image on an image carrier and a sheet due to the transport skew of the sheet, an oversized image (development-agent image) g is formed on an image carrier as shown in FIG. 17. In this case, even when the fed sheet p is skewed, a satisfactory image can be formed without any portion of the sheet p lacking the image.
In the above-described case, however, when an image having a size which significantly exceeds a transported sheet is formed on an image carrier, a large amount of development agent which has not been transferred onto the sheet is recovered by a cleaning means. In the case of an image forming apparatus which cannot reuse the recovered development agent, the recovered development agent is discarded, resulting in uneconomical use of the development agent. In addition, a container for recovery of a development agent is filled very quickly with the recovered development agent.
FIG. 18 shows a cleaning means c integrated with a container a for recovering a development agent t from a transfer belt d which is used to attach to and transport a sheet p. In this cleaning means, a greater amount of accumulation of the recovered development agent t tends to occur at sites located on the right- and left-hand sides of the sheet p in a sheet transport direction. Such accumulated development agent t is likely to partially leak, leading to cleaning failure.
To avoid this, conventionally, a means for detecting an edge position of a sheet transported toward a transfer point where an image formed on an image carrier is transferred to the sheet, is provided upstream, in the sheet transport direction, from the transfer point. A size of the sheet is detected at the edge portion by the detection means. Based on the sheet size, a size of an image to be formed on the image carrier is determined. After size determination, the image is formed on the carrier, and is then transferred to the sheet. Thus, an image having a size, which matches the size of a transported sheet, is formed on an image carrier, thereby reducing the amount of a development agent which is not transferred to the sheet and is recovered by a cleaning means. See, for example, JP 10-186951A.
In the above-described conventional technique, the image is formed on the image carrier after the sheet edge position is detected by the detection means and the size of an image to be formed on an image carrier is then determined. Therefore, the detection means needs to be placed considerably upstream, in the sheet transport direction, from the transfer point. In other words, a relatively long sheet transport path is required, which extends in the sheet transport direction from a point of detecting the edge position of a sheet by the detection means to the transfer point. Therefore, the size of the image forming apparatus becomes very large. Moreover, as the sheet transport path from the detection point of the detection means to the transfer point is increased, the time required to form an image increases accordingly.
Furthermore, as the sheet transport path from the detection point of the detection means to the transfer point is increased, the accuracy of detecting the edge position of a sheet by the detection-means is decreased. Therefore, in this case, there is the risk that a portion of an image is lost on the sheet due to the transport skew of the sheet. Conversely, when the detection means is placed closer to the transfer point, writing of an image onto an image carrier is started before detecting the edge position of a sheet. Therefore, the determination of an image size on the carrier is too late for the writing of the image.
Furthermore, a registration means (timing adjusting means) for adjusting the position of the image formed on the image carrier in relation to a transported sheet, is provided upstream, in the sheet transport direction, from the image carrier. The registration means abuts a leading edge of a transported sheet to temporarily stop the sheet. Subsequently, the registration means restarts transporting the sheet with appropriate timing. The registration means has not only the function of deciding on the timing of registering a sheet in relation to the image on the image carrier, but also a function of correcting the skew (oblique feeding) of the transported sheet.
In this case, when the detection means is placed upstream, in the sheet transport direction, from the registration means, it can be ensured that the distance of the sheet transport path from the detection point of the detection means to the transfer point does not become any longer. However, the correction of the transport skew by the registration means is not yet performed at the detection point which is located upstream, in the sheet transport direction, from the registration means. Therefore, the accuracy of detecting the edge position of a sheet by the detection means cannot be improved.
The inventors of the present application have already filed an application (Japanese Patent Application No. 2003-169429) which is directed to a technique, in which a registration means is placed upstream, in the sheet transport direction, from a detection means. In this technique, after the transport skew of a sheet is corrected, the edge position of the sheet is detected, so that the accuracy of detecting the edge position of the sheet is assured.
However, in some cases, the vertical position of a sheet is not specified upstream, in the sheet transport direction, from the registration means, resulting in a reduction in the accuracy of detecting the edge position of the sheet. For example, the vertical position of a sheet cannot be specified at a site where a plurality of transport paths (upper transport path(s) and lower transport path(s)) are merged, the site being located upstream, in the sheet transport direction, from the registration means.
FIG. 19 shows sheets p1, p2 and p3 of the same size, which are positioned at different heights, and a light emitting element h and light receiving elements i, which are used to detect the edge position of the sheet. As can be clearly seen from FIG. 19, light emitted by the light emitting element h falls on a light receiving element i which is located on a right-hand side with respect to the edge of the sheet, no matter whether the sheet to be detected is p1, p2 or p3. The position of a rightmost light receiving element i, which receives incident light from the light emitting element h, varies depending on which sheet is detected, p1, p2 or p3. Therefore, the accuracy of detecting the edge position of the sheet is poor.
FIG. 20 shows that a rod lens array j is interposed between the transported sheet and the light receiving elements i. In this case, as in FIG. 19, the position of the rightmost light receiving element i that receives incident light from the light emitting element h varies depending on which sheet is detected, p1, p2 or p3. Therefore, the accuracy of detecting the edge position of a sheet is poor.
It is thus an object of the present invention to provide a sheet detecting apparatus capable of assuring the accuracy of detecting the edge position of a sheet, even when the vertical position of a sheet is not specified; and an image forming apparatus with the same.